I. Field of the Invention
The invention relates generally to the utilization of a mixture of dimethyl ether and low molecular weight alkanes and alkenes as an improved fuel for spark ignition internal combustion engines. More specifically, the invention relates to a process and an improved fuel which produce relatively less of certain atmospheric pollutants than are produced by, for example, propane fuel.
II. Description of the Prior Art
Dimethyl ether, also known as methoxymethane or methyl ether, is of the formula CH.sub.3 --O--CH.sub.3. Having a relatively low vapor pressure as compared to compressed natural gas, dimethyl ether is readily transportable. Additionally, dimethyl ether can be economically produced in relatively small quantities, as compared to materials such as natural gas which require economies of scale associated with large cryogenic plants to be produced competitively. However, pure dimethyl ether exhibits an octane number which is too low for use in modern spark-ignition internal combustion engines.
Dimethyl ether has found acceptance as a starter fluid for gasoline fueled engines. U.S. Pat. No. 4,177,040 issued to Kaiser reports a starter fluid in aerosol spray form containing an alkyl ether, a propellant and a petroleum distillate. The use of such starter fluids in a particular engine is usually initiated when the engine is cold and discontinued as soon as the engine is operating reliably.
Various blends of dimethyl ether and alcohols have been proposed as spark-ignition engine fuels. For example, U.S. Pat. No. 4,743,272 issued to Weinberger describes a gasoline fuel substitute which includes a major amount of specified anhydrous alcohol mixtures containing methanol or ethanol and a minor amount of specified ketone or ether mixtures. While dimethyl ether-alcohol fuels are satisfactory in some respects, the alcohol components are generally toxic to humans.
Gaseous fuels such as ethane, propane, butane and mixtures thereof have been pressed into service as spark ignition engine fuels from time to time, especially during temporary fossil fuel shortages. Currently, gaseous fuels are the fuels of choice for spark ignition engines when exhaust pollutants must be minimized. For example, propane fueled fork lift vehicles are operated in warehouses where exhaust gases tend to accumulate. To the present day, however, the general public has not consistently adopted gaseous fuels, such as propane, as primary automotive fuels for routine service.
The gaseous fuels are frequently utilized as heating fuels. A fuel gas composition containing five to thirty percent by weight dimethyl ether and a balance of two or more hydrocarbons selected from the group consisting of propane, propylene, butane, and butylene is described in Japanese Kokai Patent No. JP 6086195 assigned to Idemitsu Petrochemical. The Japanese Patent states that the fuel gas composition is suitable for use in industrial plants and in large kitchens.
U.S. Pat. No. 4,892,561 issued to Levine describes fuels for internal combustion engines which contain at least fifty percent by weight of dimethyl ether. For example, the Levine Patent states that a mixture of approximately equal weights of propane and methyl ether is suitable for use as fuel for spark ignition engines. As will be explained in more detail below, the equal weights mixture endorsed by the Levine Patent has a research octane of about 73 and a motor octane number of about 55 and, therefore, cannot be utilized as a fuel for modern spark-ignition engines.
A need still exists for an improved fuel composition for spark-ignition internal combustion engines that exhibits an octane number appropriate for use in conventional automobiles. Desirably, the improved fuel can operate in existing automotive engines without major engine modifications. More desirably, the improved fuel is manufactured from materials which are widely available and produces a relatively small amount of atmospheric pollutants, as compared to propane and conventional fossil fuels.